1. Field of the Invention:
The present invention relates to powder metallurgy and specifically to iron alloys made thereby, wherein powders, which may be used, for example, in the manufacture of pressed and sintered powder metallurgical parts, are produced by a dry milling process.
2. Background Information:
Metal powders are used in powder metallurgy practice to form hardened metal parts by compressing the powder to a compacted shape and then by heating the compacted metal powder to form a coherent mass. The heating, or sintering, step may be done with or without mechanical deformation during the heating step. The present invention relates to a method for producing steel, with particular emphasis on high alloy steels and high speed tool steels. Further discussion of what constitutes these and various other steels is discussed in The Making, Shaping, and Treating of Steel, Association of Iron and Steel Engineers, 10th edition, 1985, pages 1289-1320.
In principle, alloy steels, as an example, can be manufactured simply by the mixing of iron and other elemental metal powders followed by the compacting and sintering of such mixtures. In one method of powder preparation, an elemental admixture, which may be made by simply blending an unalloyed iron powder with the alloying ingredients, forms a soft powder which is easily compactable. However, after sintering, an elemental admixture is likely to result in a product which is non-homogeneous and has poor mechanical properties.
It will be appreciated that easy compaction, or compressibility, is desired in that, in general, the higher the density of the compacted parts prior to sintering, the smaller the dimensional changes which occur as a result of the sintering process. Secondly, good compressibility results in less wear on the compaction dies used in the process of forming the hardened final product. Compressibility may be measured or defined in terms of the density achieved in a part after a given amount of applied compaction pressure. Thus, for example, a metal powder A, which was compressed at 40 tons/in.sup.2 and achieved a density of 6.9 g/cm.sup.3 would be said to have a better compressibility than powder B which, when compacted at the same pressure, 40 tons/in.sup.2, achieved a density of only 6.2 g/cm.sup.3.
Another method, for producing a partially pre-alloyed powder, includes blending an elemental admixture and immediately heating the mixture to a temperature at which the alloying ingredients are diffusion bonded to the surface of the iron powder. This method results in only marginally improved homogeneity. Further, if hard or abrasive compounds, such as carbides, are formed at the diffusion bonded interface, this method may lead to excessive die wear, an event of considerable economic disadvantage to the manufacturer.
The most common method, producing a completely pre-alloyed powder, consists of melting the desired alloy and then spray atomizing a liquid metal alloy stream to produce the powder. The powder produced by this method is very homogeneous, but is generally very hard, and also results in excessive die wear from cold-compaction difficulty. Further, alloy powders made in this manner are typically very expensive to produce and are frequently poorly sized.
Further, in the case of production of high alloy steels, attempts have been made to use mixtures of elemental powders and graphite, and also to use mixtures of iron powder with pre-melted carbides. Such mixtures result in a product with properties less than desired because sintering of this powder to near-full density has proven to be virtually impossible. In fact, it is often the case that swelling rather than densification of a pressed compact occurs when such mixtures are sintered.